This invention relates to fuel cell systems and, in particular, to fuel cell systems with waste heat recovery for production of high pressure steam.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions. High temperature fuel cells, such as molten carbonate fuel cells and solid oxide fuel cells, operate by passing a reactant fuel gas through the anode, while oxidant gas containing oxygen and other gases is passed through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series.
High temperature fuel cells typically operate at temperatures of 600° C. (1100° F.) and 1000° C. (1800° F.), depending on the type of fuel cell, and produce waste heat as part of high temperature exhaust output by the fuel cells. In addition, high temperature fuel cells typically consume 60 to 80% of the fuel fed to the fuel cell system and in conventional systems, the remaining fuel is output with fuel cell exhaust and is oxidized and converted to waste heat in an oxidizer or similar device. In conventional fuel cell systems, waste heat from the fuel cell exhaust is recovered by cooling system exhaust gas from about 398.9° C. (750° F.) to about 121.1° C. (250° F.) after pre-heating incoming fuel in a humidifier/heat exchanger. Water may be recovered from the fuel cell exhaust and the recovered water may be recycled for humidifying incoming fuel. An example of such a system is disclosed in anode exhaust water recovery U.S. Pat. No. 8,367,256, assigned to the same assignee herein.
In conventional systems, the maximum level of heat recoverable from the exhaust is limited to about 371.1° C. (700° F.) and to recover all the heat as steam, the upper temperature limit of the steam produced is about 121.1° C. (250° F.), or steam at a pressure of about 103.4 kPa (15 psig). Production of steam at a pressure higher than about 103.4 kPa (15 psig) will greatly reduce the amount of waste heat recovered. In addition, for industries that use high pressure steam of more than 4136.8 kPa (600 psig), over 70% of the waste heat in fuel cell cathode exhaust is unusable in the conventional system. Moreover, waste heat from fuel cell exhaust is recovered using a heat exchanger with a temperature approach, typically between 10.0° C. (50° F.) and 65.6° C. (150° F.). When the approach temperature is considered in the heat recovery, the amount of heat available is reduced by an additional 5 to 20%.